Power supply tracking single ended sensing scheme for SONOS memories

ABSTRACT

An apparatus and method for sensing memories is provided. In one embodiment, the apparatus includes a current sense amplifier, a column multiplexer coupled to a first input of the current sense amplifier and to a memory array, and a current reference circuit and a replica pass transistor coupled to a second input of the current sense amplifier. Other embodiments are also provided.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 13/206,378 filed Aug. 9, 2011, now U.S. Pat. No. 8,279,677, issued on Oct. 2, 2012, which is a continuation of U.S. application Ser. No. 12/151,282, filed May 5, 2008, U.S. Pat. No. 7,995,397, which claims priority to U.S. Provisional Patent Application No. 60/927,316, filed May 3, 2007, all of which are incorporated by reference herein in their entirety.

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BACKGROUND OF THE INVENTION

SONOS (Silicon Oxide Nitride Oxide Silicon) memories use single ended current sensing to reduce chip area. SONOS bi-gate memory cells include a pass transistor and a SONOS cell. The read current of the memory cell is modulated by VCC significantly (up to, for example, 8 uA/V) due to the pass transistor's VCC dependence. This causes read failures due to significant degradation of sense margin. A conventional solution uses a double ended to sensing scheme to track VCC dependence. Since there is no reference current involved and the current modulation due to pass transistors is a common mode signal for the bitline and the bitline-bar side, the VCC modulation effect on the final read speed and sense margin is removed. Unfortunately, these sort of differential read schemes involve two cells per bit of storage leading to poor area efficiency.

Another conventional solution uses a single ended sensing scheme with a pair of dummy column memory cells (one column for program cells and one column for erase cells) for every block of the array to obtain a reference current. The currents from the dummy columns of memory cells when added together can be mirrored to supply a reference cell current that is the center of program and erase current. Since the VCC modulation due to the pass transistor is common mode for the dummy columns of memory cells and the cell being accessed, the final read speed and sense margin were unaffected by pass transistor VCC modulation. Unfortunately, single ended sensing schemes that use a pair of dummy column cells to generate a reference current are less efficient in terms of chip area than a single ended sensing scheme that does not use dummy column of cells

Non-volatile memories that use single ended sensing schemes need a current sensing scheme that automatically tracks the VCC modulation of the memory cell to achieve better sense margin and high speed sensing.

BRIEF SUMMARY OF INVENTION

A SONOS memory sensing scheme includes a reference current circuit that tracks the changes in the power supply (Vcc). An equalizer of the current sense amplifier is coupled between the read out current line and the reference current line. The current sense amplifier includes data and datab (data bar) outputs which have common mode noise due to variations in the power supply voltage. The data output is a current generated from the memory cell, and the datab output is generated by the current reference circuit.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a circuit diagram of a SONOS memory sensing scheme in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a SONOS memory sensing scheme that has a reference current circuit that tracks the changes in the power supply (Vcc). An equalizer of the current sense amplifier is coupled between the read out current line and the reference current line. The current sense amplifier includes data and datab (data bar) outputs which contain the common mode noise due to variations in the power supply voltage. The data output is a current generated from the memory cell, and the datab output is generated by the current reference circuit.

FIG. 1 is a circuit diagram of a SONOS memory sensing scheme 10 in accordance with one embodiment of the invention. The sensing scheme 10 has three main elements: a non-volatile (NV) memory array 12; a current sense amplifier 14; and a current reference circuit 16. The NV memory array 12 includes any suitable number of columns of memory cells, of which only a portion of one column is shown for purposes of illustration and not limitation. The NV memory array 12 shows a column of pass transistors 18, 20, 22, which are labeled, MPASS0, MPASS1, MPASSn (where n can be any suitable number). The gates 24, 26, 28 of the pass transistors 18, 20, 22 are coupled to the word lines WL0, WL1, WLn, respectively. The drains 30, 32, 34 of the pass transistors 18, 20, 22 are coupled to the sense line (SL) 36. The sources of the pass transistors 18, 20, 22 are coupled to the drains of the SONOS transistors 38, 40, 42. The SONOS transistors 38, 40, 42 are labeled MSONOS1, MSONOS2, MSONOSn. The gates 44, 46, 48 of the SONOS transistors 38, 40, 42 are coupled to SONOS word lines, WLS0, WLS1, WLSn, respectively. The sources of the SONOS transistors 38, 40, 42 are coupled to the bit line (BL). A column multiplexer 52 couples the bit line 50 to the current sense amplifier 14, although the column multiplexer 52 is capable of coupling one of a plurality of bit lines in the NV memory array 12 to the current sense amplifier 14.

The current sense amplifier 14 includes PMOS transistor (MP1) 54 with a source 56 coupled to the power supply voltage (Vcc) 58. The gate 60 of transistor 54 is coupled to an enable signal (saenb). The drain 62 of transistor 54 is coupled to the first nodes of a pair of loads 64, 66. An equalization circuit 68 is coupled between the second nodes of the loads 64, 66. The equalization circuit 68 is controlled by an enable signal (saeq) 70. The load 64 is coupled to the data output line 72 and to the column multiplexer 52, and the current from the memory cell (Icell) is carried by this conductor. The load 66 is coupled to the datab output line 74 and to the drain 76 of NMOS transistor (MN2) 78. The source 80 of transistor 78 forms the signal Iref2 and is coupled to the drain 82 of NMOS transistor (MPASSR) 84. The source 86 of transistor 84 is coupled to the low voltage of the power supply (VSS) 88. The gate 90 of transistor 84 is coupled to the high voltage of the power supply (VCC) 58. The gate 92, labeled nbias, of transistor 78 is coupled to the current reference circuit 16.

The current reference circuit 16 includes a current source (Iref) 94 coupled to the power supply voltage 58. The output of the current source 94 is coupled to the drain 96 and gate 98 of NMOS transistor (MN1) 100. The source 102 of transistor 100 is coupled to the low voltage of the power supply (VSS) 88. Thus, the current reference circuit 16 is configured to create a suitable bias at the gate 98 of transistor MN1 100 for a given reference current (Iref1).

The current sense amplifier 14 shown in FIG. 1 takes in the current (Icell) from the NV memory array 12 and compares it to the reference current (Iref2) and outputs a differential voltage output voltage (data−datab) which can be further amplified by downstream voltage amplifiers. When the input saenb 60 is low, it turns on the current sense amplifier 14 by turning ON transistor MP1 54. Input saeq 70 comprises an enable to the equalization circuit 68 to help precharge and equilibrate the differential nodes prior to turn ON of the current sense amplifier 14. The loads 64, 66 serve as the loading devices for the current sense amplifier 14. The loads 64, 66 can be designed using, for example, diode connected devices, resistors, current mirror loads, biased transistors, or the like. Transistor MN2 78 comprises a mirror of transistor MN1 100, but with its source degenerated by the Vds drop of transistor MPASSR 84. Transistor MPASSR 84 comprises a replica of the PASS transistors 18, 20, 22 in the NV memory array 12 which sees a power supply bias on its gate during a read operation.

For purposes of illustration and not limitation, during a read operation row0 is selected by taking WL0=VCC, SL=0 and WLS0=0. The column multiplexer 52 is turned ON by a column address. This causes the current Icell to flow through the bitlines, colmux and the sense amplifier loads to VCC. Since WL=VCC, the current Icell is modulated by a change in the power supply voltage VCC. In the reference side, the Iref1 current is set by the current source 94 to give a bias that will create a current that is, for example, approximately 0.5*(Iers+Iprg), where Iers is the worst case erase current and Iprg is the worst case program current. Gate 92 of transistor MN2 78 gets this bias voltage set at the gate 98 of transistor MN1 100, but also has its source degenerated by a replica PASS transistor MPASSR 84. The Vds drop across MPASSR 84 degenerates the source 80 of transistor MN2 78 such that Iref2 is also modulated due to power supply variation, and, thus, tracks the cell current modulation. Since the current is affected on both sides of the amplifier, this becomes common mode and results in very good sense margin and read speeds across power supply variations.

The new amplifier according to exemplary embodiments of the present invention achieves robust and accurate power supply tracking through the use of a replica PASS transistor 84 in the current sense amplifier 14. The present new sense read scheme is area efficient, because it does not use an entire column of dummy cells. In addition, the new scheme achieves good sense margin and read speeds even for wide variation of power supply voltage.

Exemplary embodiments of the present invention can be used in conjunction with any suitable type of memory, such as, for example, non-volatile SONOS memory technologies or products, or other like memories. Each of elements of the circuit 10 can be comprised of any suitable type of electrical or electronic circuit, component or device that is capable of performing the functions associated with the respective element. For example, each of the transistors illustrated in FIG. 1 can be comprised of any suitable type of transistor or other like device.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alterations, modifications, and variations in the appended claims. 

What is claimed is:
 1. A sensing apparatus for memories, comprising: a current sense amplifier; a column multiplexer coupled to a first input of the current sense amplifier and to a memory array; and a current reference circuit coupled to a gate of a transistor in a second input of the current sense amplifier to couple a reference current (Iref) to the second input, the transistor comprising a source coupled to a low voltage supply (VSS) through a replica pass transistor, wherein the replica pass transistor comprises a gate coupled to VCC, and degenerates the source of the transistor to modulate Iref to track changes in VCC, and wherein the transistor in the second input is a mirror transistor mirroring a current through a bias transistor in the current reference circuit.
 2. The apparatus of claim 1, wherein the memory array comprises non-volatile memory cells.
 3. The apparatus of claim 1, wherein the memory array comprises Silicon Oxide Nitride Oxide Silicon (SONOS) memory cells.
 4. The apparatus of claim 1, wherein the current sense amplifier comprises a single-ended current sense amplifier.
 5. A method comprising: coupling a cell current (Icell) from a memory cell to a first input of a current sense amplifier, wherein Icell is modulated by a change in a high voltage supply (VCC) to the memory cell; and coupling a bias voltage from a current reference circuit to a gate of a transistor in a second input of the current sense amplifier to couple a reference current (Iref) to the second input, the transistor comprising a source coupled to a low voltage supply (VSS) through a replica pass transistor, wherein the replica pass transistor degenerates the source of the transistor to modulate Iref to track changes in VCC, wherein the transistor in the second input is a mirror transistor mirroring a current through a bias transistor in a current reference circuit.
 6. The method of claim 5, wherein coupling the bias voltage from the current reference circuit to the gate of the transistor comprises coupling a bias voltage selected to provide an unmodulated Iref through the mirror transistor that is substantially centered between a program current and an erase current.
 7. The method of claim 6, wherein coupling Icell to the first input comprises coupling Icell to a first load in the first input, and coupling Iref to the second input comprises coupling Iref to the second input through a second load.
 8. The method of claim 7, wherein the current sense amplifier further comprises an equalizer circuit coupled between the first load and the second load, and wherein the method further comprises reducing with the equalizer circuit a differential between the first load and the second load prior to coupling Icell to the first input or coupling the bias voltage to the gate of the mirror transistor in the second input.
 9. The method of claim 5, wherein the memory cell comprises a non-volatile memory cell.
 10. The method of claim 5, wherein the memory cell comprises a Silicon Oxide Nitride Oxide Silicon (SONOS) memory cell.
 11. The method of claim 5, wherein the current sense amplifier comprises a single-ended current sense amplifier. 